1. Field of the Invention
This invention relates to heat exchanger evaporators and condensers, particularly those used in HVAC applications. In particular, the invention relates to a shell and tube type evaporator, where the refrigerant flows through the tubes and evaporates, while a heat transfer fluid flows through the shell and is cooled by the evaporating refrigerant. In a preferred embodiment, the evaporator is a component of a refrigeration system which can be used for cooling large quantities of water. This invention relates to an apparatus and method for increasing the heat transfer rate of these types of heat exchangers.
2. Description of the Related Art
Refrigeration systems of the type used to cool large quantities of water typically include a heat exchanger evaporator having separated passageways. One passageway carries refrigerant, and another carries the heat transfer fluid to be cooled, usually water. As the refrigerant travels through the evaporator, it absorbs heat from the heat transfer fluid and changes from a liquid to a vapor phase. After exiting the evaporator, the refrigerant proceeds to a compressor, then a condenser, then an expansion valve, and back to the evaporator, repeating the refrigeration cycle. The fluid to be cooled passes through the evaporator in a separate fluid channel and is cooled by the evaporation of the refrigerant. The fluid can then be routed to a cooling system for cooling the spaces to be conditioned, or it can be used for other refrigeration purposes.
It is desirable to optimize the heat transfer rate between fluids flowing through a heat exchanger, particularly large heat exchangers used in heating and air conditioning systems. A number of approaches have been proposed to improve the heat transfer characteristics of evaporators and condensers. One generally known approach is to create an electric field on a heat transfer surface in order to improve heat transfer. The use of an electric field to improve the heat transfer of convection heat transfer in a liquid is generally referred to as the electrohydrodynamic effect or EHD. Applications of this approach are disclosed in U.S. Pat. No. 4,651,806 to Allen et al., U.S. Pat. No. 5,072,780 to Yabe, and U.S. Pat. No. 5,769,155 to Ohadi et al.
While the general concept of EHD is known, the systems described in the prior art are cumbersome and difficult to install, and their efficiency is suspect.
Therefore, there is a need for a simple, small, and efficient apparatus and method for increasing the heat transfer rate of heat exchangers, particularly ones used to evaporate or condense refrigerants in HVAC systems.
The object of the present invention therefore is to provide improved heat exchanger methods and systems. Another object is to provide improved heat exchangers for HVAC applications that are made of inexpensive components, are economical to build, and are more compact than conventional heat exchangers.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, the invention includes a heat exchanger assembly for use in an HVAC system. The heat exchanger assembly includes: a shell-type casing; a plurality of tubes located inside the shell-type casing, each of the tubes having an outer surface and an irregular inner surface; and a plurality of electrodes. Each electrode is located inside one of the corresponding tubes to create a space between the electrode and the inner surface of the tube. A first fluid is located in each space between the electrodes and the inner surfaces of the tubes. A second fluid is located in a space between the tubes and the shell-type casing to flow across the outer surfaces of the tubes. A voltage is applied on the electrodes in order to increase the rate of heat transfer between the first fluid and the second fluid.
In another aspect, the invention includes an evaporator for transferring heat between a heat transfer fluid flowing over an outer surface of a tube and a refrigerant flowing through the tube. The evaporator includes: a tube including a first end, a second end, an inner surface with surface irregularities, and an outer surface; and an electrode located inside and spaced from the tube. The electrode includes an outer surface and has a voltage applied to produce an electric field. A refrigerant flows through the tube in the space between the electrode and the inner surface of the tube. A heat transfer fluid flows over the outer surface of the tube. The electric field produced by the applied voltage of the electrode presses the refrigerant against the surface irregularities on the inner surface of the tube in order to increase the heat transfer rate between the refrigerant and the heat transfer fluid.
In a further aspect of the invention, the invention includes a heat exchanger assembly. The heat exchanger assembly comprises: a tube including a first end, a second end, an inner surface and an outer surface; and an electrode located inside and spaced from the inner surface of the tube. The electrode includes an outer surface and has a voltage applied to produce an electric field. A first fluid flows in the space between the outer surface of the electrode and the inner surface of the tube, and a second fluid flows along the outer surface of the tube. One of the inner surface of the tube and the outer surface of the electrode includes surface irregularities, the effect of the electric field on the surface irregularities being an increase in the heat transfer rate between the first fluid and the second fluid.
In a yet further aspect of the invention, the invention includes a method of exchanging heat between a heat transfer fluid and a refrigerant in a shell and tube heat exchanger. The method includes the steps of: providing an electrode inside a hollow tube; flowing the refrigerant through the hollow tube in a space between the electrode and hollow tube, the refrigerant flowing along an inner surface of the hollow tube having surface irregularities; flowing the heat transfer fluid around an outer surface of the hollow tube; and applying a voltage to the electrode to create an electric field, the electric field pressing the refrigerant against the surface irregularities of the hollow tube to increase the heat transfer rate between the refrigerant and the heat transfer fluid.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.